Referring to FIG. 1, a conventional backlight module 1 includes a light guide plate 12 having a top surface 121 and a lateral surface 122 transversely connected to the top surface 121, a light source unit 11 disposed on the lateral surface 122 of the light guide plate 12, a first diffuser plate 13 disposed on the top surface 121 of the light guide plate 12, a first prism plate 14 disposed on the first diffuser plate 13 and opposite to the light guide plate 12, and a second diffuser plate 15 disposed on the first prism plate 14 and opposite to the first diffuser plate 13. Light emanating from the light source 11 enters the light guide plate 12 through the lateral surface 122 and exits from the second diffuser plate 15 through the light guide plate 12, the first diffuser plate 13, the first prism plate 14 and the second diffuser plate 15. The first prism plate 14 includes a plurality of first microstructure members 141 extending in a direction parallel to the lateral surface 122 of the light guide plate 12 so as to refract the light entering the first prism plate 14 to be converged in the direction perpendicular to the lateral surface 122. Thereafter, the refracted and converged light will enter and be diffused by the second diffuser plate 15 and then exit from the backlight module 1 to provide a light field with improved luminous intensity.
Referring to FIG. 2, in order to further improve the luminous uniformity of the light, the conventional backlight module 1 further includes a second prism plate 16 that is disposed between the first prism plate 14 and the second diffuser plate 15. The second prism plate 16 includes a plurality of spaced-apart second microstructure members 161 that extend in a direction perpendicular to an extending direction of the first microstructure members 141 of the first prism plate 14. However, the design of the conventional backlight module 1 advantageously improves the luminous intensity of a central portion thereof but adversely restricts the viewing angles. Hence, when the conventional backlight module 1 is applied to a display apparatus, the performance of the display apparatus is adversely affected by decreased viewing angles of the backlight module 1.